Efficiency dehumidifier drier with reversible airflow and improved control

ABSTRACT

An apparatus and process including a heat sink exchanger ( 26 ) to cool and condense liquid out of a drying gas with a heat transfer surface arranged to exchange heat with a first sub-stream of the drying gas and a heat source heat exchanger ( 27 ) arranged to exchange heat with a second sub-stream of a drying gas and arranged in a functionally parallel configuration with said heat sink heat exchanger ( 26 ) so that each of said drying gas sub-streams exchanges heat with one of the two said heat transfer surface per cycle through the heat exchange system and a gas movement device ( 35 ) for propelling the drying gas through the heat exchanger system in either a forward or reverse flow path direction. The apparatus and process can also include controlling the amount of heat rejected from apparatus ( 26 ) based on maintaining the wet bulb of the drying gas nominally constant and controlling the amount of refrigerant in the heat exchanger circuit based on maintaining the dry bulb temperature of the drying gas within certain limits.

FIELD OF THE INVENTION

The present invention relates to the drying of materials using a heatpump or heat integrated dehumidifier system to move energy to evaporateliquid from wet material. It has particular application to the drying oftimber but is also well suited for numerous other drying processes.

BACKGROUND TO THE INVENTION

Most milled timber and many other materials dried on an industrial scaleare currently dried by kilns operating on a heat-and-vent principlewhere ambient air is heated by indirect contact with steam or by someother high temperature heat source, passed over the timber or othermaterial to be dried, and vented back to the atmosphere. This process isoften relatively rapid but energy inefficient. Alternative dryingmethods using heat pump based drying systems have been generally knownin industrial applications including timber drying for a number of yearsbut they have had varying degrees of success based on limitations inperformance, control and efficiency.

References to the use of heat pump refrigeration cycles in clothesdrying date back to the 1940s in U.S. Pat. No. 2,418,239. Because of thecomplexities of both the drying process itself and the operation of anominally closed loop drying system driven by a heat pump dehumidifier,there has been a need to provide active control of the process to bothmaintain its peak efficiency throughout the drying process and to ensurethe quality of the dried product. This need is complicated by the factthat the control of a heat pump dehumidifier system and the dryingprocess parameters themselves are linked by multiple feed-back processesthat are fundamentally different from the more commonly practiced butless efficient heat-and-vent drying systems. Another of the key featuresof heat pump drying systems has been their inherent energy efficiency.The energy crisis of the 1970s focussed attention on energy efficiencyand several items of prior art from just after this period reflect thisfocus.

One further problem that has developed more recently as part of the highdrying speed is that the characteristics of the dried material are lesssuitable to the end users of the dried product. In the case of timber,these difficulties include kiln brown stain and internal checking.(Kreber, Haslett, McDonald, 1999; Bannister, Carrington, Chen, 2002) Asa result, slower lower temperature drying methods have increased infavour because the loss of production speed is compensated for by thebetter quality dried product. (Bannister, Carrington, Chen, 2002)

Another problem is the uneven drying that results when the hot dryinggas, typically air, is passed over the material to be dried in a singledirection throughout the process. Material that is exposed to the hotdrying gas first dries more quickly than the material further downstreamin the configuration and can become over-dry on one side and under-dryon the other, with adverse quality implications. This problem isnormally avoided in heat-and-vent kilns by reversing the drying gas flowdirection (Keey, Langrish, Walker, 2000). Because of the fundamentalsimplicity of the heat-and-vent process, the airflow can easily bereversed periodically. One such system is that described by Rosenau inU.S. Pat. No. 4,356,641. Here a reversible air flow configuration isaugmented by a switchable control system to better accommodate thereversible air flow. Another such system is described in U.S. Pat. No.5,276,980 by Carter and Sprague which uses a complex air handling systemwith multiple drying chamber sections. However this problem of unevendrying is still present in heat pump driven kilns because the heat pumpdesign has so far prevented any efficient reversing of the drying gasflow during operation.

Another problem that is present with heat-and-vent kilns is theirfundamentally poor energy efficiency. The efficiency specificallydecreases when the operating temperature is lowered in response toquality requirements. The productivity also decreases as the temperatureis reduced (Keey, Langrish, Walker, 2000). Although they can sometimesbe driven with waste heat systems, low temperature heat-and-vent systemstypically require a high capital investment relative to theirproductivity which diminishes their attractiveness. (Bannister, Chen,Grey, Carrington, Sun, 1997)

Despite a higher inherent efficiency relative to heat and vent systems,heat pump based drying systems have also focussed their development onfurther improving this inherent efficiency through a variety ofdifferent improvements. One example of previous methods to address theproblem of improving energy efficiency over a wider range of operatingconditions is described by Lewis in U.S. Pat. No. 4,250,629. As withmost efficient heat pump systems this is a closed loop process whichheats the air before it enters a drying chamber and then removes some ofthe moisture from the air after it leaves the drying chamber before itis largely recirculated and goes through the process again. This systemhas the specific capability of air bypass controls on the heat pumpallowing independent control of airflow through or around the heat pumpevaporator to improve the range of temperatures over which the heat pumpcycle can operate efficiently. However, the controls and louvers in sucha system will need to be positioned in the active drying gas flow pathwhich tends to increase the pressure drop through the drying gas circuitwhich cuts into the efficiency gains for the process. Anotherunsatisfactory aspect is that having critical mechanical moving parts inthe kiln reduces system reliability. Louver type airflow controls tendto fail in the aggressive environment and this can result in damage tothe product or the heat pump.

An example of improvements specifically targeted at efficiency is putforward by Thompson in NZ 213728. He describes a heat pump timber dryingprocess and apparatus which uses multiple chambers and a heat reservoirto improve drying efficiency. Although effective from an efficiencyperspective, the capital cost and operating difficulties associated withsuch a system are a significant disadvantage.

Goodwin and Hogue in U.S. Pat. No. 5,138,773 address the energyefficiency aspects of timber drying from the universal perspective ofdry wall insulation materials for the kiln chamber. Their apparatus forinsulation will improve the efficiency of both heat pump and non-heatpump based drying systems.

Goodwin in U.S. Pat. No. 5,595,000 proposed efficiency improvements to apartially recirculating dehumidification system which has someapplicability to a heat pump driven system but does not specificallyindicate such an application. These efficiency improvements are based onadding a connected set of heat exchangers to recover sensible heat moreefficiently from the drying gas stream. A first heat exchanger removessensible heat from the drying gas medium upstream of a second cold heatexchanger condensing moisture from the drying gas medium and then themajority of that heat removed in the first exchanger is returned to thedrying medium in a third exchanger downstream of the second coldexchanger. Blundell (1979) has described the use of such a heat recoverysystem for increasing the drying efficiency of a heat pump dehumidifier,and data on the performance of such a dryer was presented by Bansal,Bannister and Carrington (1997).

U.S. Pat. No. 6,209,223 by Dinh describes a grain drying system with aheat pump configuration which employs additional recovery of waste heatfrom an internal combustion engine in series with the heat pumpcondenser as a means of heating the drying gas medium more efficiently.

All of these efforts to improve the efficiency of heat pump anddehumidifier drying processes indicate a clear and continuing focus onthis inherent problem with all drying systems. Just as with the effortto reduce the capital cost of a drying apparatus, the effort to improveits efficiency is never completely finished. As such, if an efficiencyimprovement is of low cost and high value relative to existingtechnology, it will be a useful invention.

As implied by the work of Lewis in U.S. Pat. No. 4,250,629 it has becomeapparent that the performance of a heat pump drier depends critically onthe temperature and humidity of the recirculating drying gas medium. Asdescribed by Carrington, Bannister, Bansal and Sun (1995), theperformance of a traditional heat pump drying system can be optimisedfor a particular set of conditions, but this set of operating parametersis likely to be sub-optimal at other conditions.

Aspects of this performance problem were noted as early as 1943 in U.S.Pat. No. 2,332,981 by Anderson specifically relating to railroad car airconditioning systems. He worked to address this through an evaporatorwith an adjustable surface area where the configuration is designed tohave all of the air to be cooled flowing across all of the activeevaporator area for all of the area variations. While effective in theair conditioning application, this functionally series configuration isnot flexible enough to work effectively in heat pump dryingapplications.

U.S. Pat. No. 4,596,123 by Cooperman attempts to address the performanceproblem caused by varying heat source conditions for a heat pump heatingsystem through the use of a segmented evaporator to deliver “asubstantially constant quantity of extracted heat to the condenser viathe refrigerant substantially independently of the environmentaltemperature” based on sensing the pressure of the refrigerant betweenthe evaporator and the compressor or the electric current demand of thecompressor. Cooperman's work clearly improves the performance of a heatpump system with an ambient air heat source where a constant quantity ofheat is required at the condenser but this is not suitable for a heatpump drying system which has widely varying requirements on the heatpump condenser side as well. There is no capacity to vary the heatoutput through the condenser or the flow of refrigerant through it.

U.S. Pat. No. 5,253,482 by Murway also has a multiple section evaporatorheat pump system with an ambient air heat source to maintain a constantrate of heat recovery to the high temperature sink similar to the systemby Cooperman only based on maintaining a precisely set saturationpressure and temperature of the refrigerant in the circuit. As forCooperman's work, maintaining a strictly constant supply of heat is notwell suited for drying applications. Also, there is no capacity to varythe heat output through the condenser or the flow of refrigerant throughit.

U.S. Pat. No. 6,138,919 by Cooper and Rawhouser also propose a multiplesection evaporator system with an ambient air heat source for heatingswimming pools similar to both Murway and Cooperman's systems. Again,there is no capacity to vary the heat output through the condenser orthe flow of refrigerant through it.

The difficulty with these last three attempts to improve heat pumpperformance through evaporator area control is that they arespecifically designed for use in open environments and to provide aconstant supply of heat through the condenser. In drying applications,there are two key differences relevant to the present invention. Thefirst is that the heat source stream is the drying gas flow and theremust be condensation of moisture to remove the moisture vapour from theprocess which requires a new configuration for the variable evaporatorarea. The second is that the heat flow required from the condenser dropsoff significantly as the material dries. This makes such open heatsource, constant heat supply rate designs present in the prior art illsuited for drying applications. It is therefore, one object of thepresent invention to provide method and means to improve the efficiencyand performance of a heat pump dehumidifier suitable for use in thevariable demand conditions of a material drying system.

Another problem with many existing heat and vent kiln systems is thehighly prominent vapour plume associated with the warm wet drying gasvented from the unit. In lumber drying these emissions typically containvolatile organic products, including hazardous air pollutants such asformaldehyde. The concentration levels of formaldehyde emissions fromhigh temperature Pinus radiata kilns can be high compared withwork-place emission standards in New Zealand (Keey, Langrish, Walker,2000). Even when it does not contain polluting components, the vapourplume is a clear indication of industrial activity that has becomeundesirable in many situations.

Although heat pump based systems with essentially closed loop drying gasconfigurations essentially solve the plume problem, they do not possessother desirable characteristics of the heat and vent systems.

One of these specific characteristic problems with heat pump drivensystems is the difficulty in reversing the drying gas flow in the dryingchamber to promote even drying of the material as is done forheat-and-vent driers. This problem results from the specificconfiguration of the heat pump condenser, which condenses the heat pumpworking fluid and heats the recirculating drying gas stream, and theheat pump evaporator, which evaporates the heat pump working fluid andremoves some of the moisture from the recirculating drying gas stream bycooling it and inducing water condensation. With the typical sequentialseries configuration in the existing heat pump and heat integrateddehumidifier technology, moisture laden drying gas enters therefrigerant evaporator and loses some of its moisture by condensationbefore it then passes to the refrigerant condenser to be reheated.Drying gas flow cannot be reversed in this system without dramaticallyreducing the drying capacity and efficiency, since it would result inthe evaporator wastefully recooling part of the heated drying gas fromthe condenser and removing less moisture relative to the amount of heatremoved. Because drying gas flow reversal in the existing dehumidifiersis not practical, some dehumidifier timber kiln operators have attemptedto overcome the problem of uneven drying by leaving the kiln fansrunning for long periods without running the dehumidifier, in order toeven-up the moisture content of the boards in different parts of thestack. But this reduces the kiln production rate and efficiency and thusreduces its profitability.

U.S. Pat. No. 4,182,048 by Wolfe and Hinton describe a general methodfor drying wood in reversible air drying gas flow with a heat pumpsystem but provide no details of the method by which the heat pumpevaporator and condenser heat or cool the air stream to provide thedehumidification. The only specifics they provide relate to thetemperature and humidity of the drying air in the wood drying chamberand the time spent at those nominal conditions. Without any details ofthe method of the heat pump dehumidification of the air or any claimsrelating to an apparatus to conduct their method, the problem remains.

The reversible air circulation system in U.S. Pat. No. 5,276,980 byCarter and Sprague is nominally applicable to heat pump systems butstill has problems with its application. In this system the heat pump islocated outside the kiln chamber. One difficulty with this system is thecapital cost of the complex air drying gas ducting system required forair off-take and return, and the cost of operating the fans needed todeliver the required air volumes. Similarly they provide no details onany associated heat pumping method or apparatus.

U.S. Pat. No. 6,021,644 by Ares and Lakdawala has a related heat pumpconfiguration where they reverse the flow direction of the refrigerantworking fluid but this is focussed on defrosting the heat pumpevaporator coils rather than providing a reversing drying gas flow forindustrial drying processes.

U.S. Pat. No. 6,209,223 by Dinh describes a grain drying system with anoptional heat pump configuration used to provide a hot air drying gasstream. The system they propose is based on single pass operation forthe air drying gas. In their preferred embodiment, the drying gas isfirst cooled and then reheated in sequence. The system has no capabilityof reversible drying gas flow nor does it provide for any recirculationor regeneration of the drying gas stream in a closed or semi-closed loopconfiguration. Thus it must continually take in a full stream of freshambient drying gas and heat it up to the operating conditions. As aresult, in all but extremely high temperature ambient conditions, theirunit is not able to operate as efficiently as a closed loop system.

There also have been several attempts to address this problem in theheat-and-vent technology that are generally relevant to this invention.It is important to note that they only relate to drying processes wherethe drying gas enters the process, is heated to increase its moistureuptake capacity, passes across the material being dried to take up someof the moisture, and a substantial portion of the moisture laden gas isthen vented from the process, while the remaining balance is typicallyreheated and re-circulated into the material inlet air-stream. Thisheat-and-vent process is fundamentally different from a nominally closedloop system where the drying gas is cooled and its vapour-phase moisturecontent partially condensed to increase its moisture uptake capacity,heated to provide energy for further moisture evaporation, and passedacross the material to be dried where it takes up more moisture beforeit is recycled through the process again and again with only minor purgeand make-up streams removed and added to control various gascompositions. As such, these other attempts to control heat-and-ventprocesses do not address the problem for high efficiency heat pumpdriven systems with nominally closed loop drying gas recycle streams.

One such attempt to improve heat-and-vent drying control has been putforward by Rosenau in U.S. Pat. No. 4,356,641. This patent describes aheat-and-vent drier focussed on maintaining an acceptable constant rateof drying for lumber. This system provides control by sampling themoisture content of the lumber to determine the rate of change ofmoisture content in the lumber stack, and then adjusts the rate ofdrying if required by varying the drying gas wet bulb temperature setpoint. The difference between the actual wet bulb and the desired setpoint controls the rate of drying gas venting or a steam sprayinjection. The dry bulb temperature is also used to help control thesystem in that the difference between the measured value and the setpoint is used to control the heat input rate from the kiln heater. Asidefrom not considering the option for a recirculating drying gas systemdriven by a heat pump dehumidifier, the method changes the wet bulbtemperature of the drying gas to control the drying rate which caninvolve other difficulties discussed further in the detailed descriptionof the invention.

Another example of control technology applied to traditionalheat-and-vent timber kiln drying efficiency was put forward by Gelineauand Kinney in U.S. Pat. No. 4,599,808. Their system improves dryingefficiency by trying to maintain a constant rate of drying throughmaintaining a constant dry bulb temperature difference in the drying gasflow before and after it passes over the material being dried. Again,this method does not address the operational limits and opportunitiesassociated with a heat pump driven system.

Another example of control technology applied to the traditionalheat-and-vent timber kiln process was put forward by Moren and Ab inU.S. Pat. No. 5,940,984. They use the temperature difference across thetimber stack as the measured variable to control the process. Althoughthey follow a schedule with a nominally constant wet bulb temperature inthe drying gas during the main part of the drying process, they decreasethe wet bulb temperature at the end of the process which would causedifficulties with a heat pump dehumidification process. Since there isno heat pump dehumidification addressed in this work, it is also notable to address the efficiency issues associated with the simultaneouscontrol of a heat pump dehumidifying system. In addition, since heat andvent systems typically run with higher heat transfer per pass of dryinggas, the drying gas temperature change as it passes over or through thematerial being dried is higher and more easily used as a controlvariable. With the lower temperature differences present in heat pumpdehumidification systems, this is not a practical measured variable forcontrol and thus the problem remains.

One example of previous methods to address the problem of heat pumpdrier control is described by Lewis in U.S. Pat. No. 4,250,629. As withmost efficient heat pump systems this is a closed loop process whichheats the air before it enters a drying chamber and then removes some ofthe moisture from the air after it leaves the drying chamber before itis largely recirculated and goes through the process again. This controlsystem is focussed on increasing the range of drying gas dry bulbtemperatures over which the system can operate and provides this controlby measuring the dry bulb temperature of the drying gas in the vicinityof the heat pump evaporator and then acts to vary the amount of airdrying gas bypass around the heat pump evaporator according to this drybulb temperature. The dry bulb temperature of the drying gas is themeasured variable used to manipulate the amount of heat rejected fromthe kiln chamber and thus is related to the control of the overalldrying rate of the system. This method has the disadvantage that, as theproduct dries and the wet bulb temperature falls in response, thedehumidifier drying capacity also falls, typically reducing the dryingrate and the drying efficiency unnecessarily. Such systems requireactive intervention to repeatedly adjust the dry bulb set point as thedrying process advances to sustain the productivity and efficiency ofthe system. The timing of these adjustments is critical. If thetemperature is increased too early, the product may be damaged and losevalue. If the adjustment takes place too late, the drying time will beunnecessarily extended, with accompanying loss of productivity andincreased drying costs.

Another example of heat pump drier control has been put forward byThompson in NZ 213728. He describes a heat pump timber drying processand apparatus which uses multiple chambers and a heat reservoir with ageneralised control system based on dry bulb and wet bulb temperaturesof the drying gas stream. This control system is focussed on maintaininga desired dry bulb temperature and relative humidity in the dryingchamber rather than focussing on optimising the performance of the heatpump in the context of the drying process. The control strategy suffersfrom similar limitations to those of Lewis in U.S. Pat. No. 4,250,629and as a result does not fully address the problem of integrated controlof the drying process and the heat pump system.

In heat pump dehumidifier drying applications, the drying gas flow isprimarily or fully in recirculation and there is a key difference withrespect to the prior art relevant to the present invention. Thisdifference relates to the fact that the heat flow required from thecondenser drops off significantly as the material approaches its finaldry state under batch drying conditions as practiced in the prior art.The corresponding method of heat pump and process control must thereforeconsider the best way to address this situation in the context of boththe limitations and capabilities of the heat pump as well as thecharacteristics of the moisture release from the material being dried.This makes such single focus control designs present in the prior artopen to the improvements proposed in the present invention. It istherefore, an object of the present invention to provide an improvedcontrol method and means to increase the efficiency and performance of aheat pump dehumidifier suitable for use in the variable demandconditions of a material batch drying system.

Thus there is a continuing need to improve both the control andefficiency of heat integrated and heat pump based drying systems as wellas a need to economically and efficiently address the problems with thesingle direction drying gas flow inherent with heat pump and heatintegrated systems that lead to unevenly dried product.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a heat pump and/or heatintegrated drying apparatus and/or process which provides a clearimprovement to one or more of the current practice problems of: theinability to effectively and efficiently provide even drying throughreverse flow in heat integrated or heat pump driven drying apparatusand/or processes, the limited efficiency of existing heat integrated andheat pump driven drying apparatus and/or processes, and/or the limitedability to provide control of heat pump driven drying apparatus and/orprocesses which efficiently or effectively addresses the integration ofthe operation of the heat pump and the inherent nature of the dryingprocess

In one aspect the present invention may be said to consist of a heatexchange apparatus operable in a drying apparatus with reversible dryinggas flow including a cold heat exchanger and a hot heat exchangerarranged such that during operation the heat exchangers lie in afunctionally parallel configuration relative to the drying gas flow,whereby a first sub-stream of the drying gas flow substantiallyexchanges heat with only the cold heat exchanger, and a second substream of the drying gas flow substantially exchanges heat with only thehot heat exchanger.

More particularly the invention may be said to comprise a heat exchangesystem for a drying apparatus, including:

a heat sink heat exchanger to cool and condense liquid out of a dryinggas, with a heat transfer surface arranged to exchange heat with a firstsub-stream of the drying gas, and

a heat source heat exchanger to heat the drying gas, with a heattransfer surface arranged to exchange heat with a second sub-stream ofthe drying gas, and arranged in a functionally parallel configurationwith said heat sink heat exchanger so that each of said drying gassub-streams exchanges heat with one of the two said heat transfersurfaces per cycle through the heat exchange system, and

a gas movement device for propelling the drying gas through or aroundthe heat sink and heat source heat exchangers in either a forward or areverse flow path direction.

Preferably at least part of the heat source heat exchanger is acondenser in a heat pump system. Preferably at least part of the heatsink heat exchanger is an evaporator in the heat pump system.

Preferably the heat exchange system is arranged to heat the drying gasto a temperature between 25 and 90C.

Preferably the system includes a gas flow path arranged to substantiallymix the two gas streams after they have passed through or around saidheat sink and heat source heat exchangers.

Preferably the system includes a control system arranged to reverse thedrying gas flow direction based on any one or more of drying time,moisture content, wet or dry bulb temperature or relative humidity ofthe drying gas, or integrated amount of moisture removed from the dryinggas.

In one form said heat sink heat exchanger contains a heat sink medium tocool and condense liquid out of the drying gas, with a heat sink heattransfer surface comprising two or more sections connected in afunctionally parallel configuration with each other arranged to exchangeheat with two or more substreams of the drying gas so that each dryinggas sub-stream exchanges heat with no more than one of the two or moresaid heat sink heat transfer surface sections per cycle through the heatexchange system.

A control system is arranged to control the flow of heat sink medium inthe heat sink heat exchanger sections and increase, decrease, turn on,and/or turn off the flow of heat exchange medium independently in eachof the heat sink heat exchanger sections.

In another aspect the present invention may be said to consist of adrying apparatus with reversible drying gas flow including a dryingchamber for material to be dried and a heat exchange apparatus, whereinthe heat exchange apparatus includes a cold heat exchanger and a hotheat exchanger arranged such that during operation the heat exchangerslie in a functionally parallel configuration relative to the drying gasflow, whereby a first sub stream of the flow substantially exchangesheat with only the cold heat exchanger, and a second sub stream of theflow substantially exchanges heat with only the hot heat exchanger.

In another aspect, the present invention may be said to consist in aprocess of and/or apparatus for drying a material including: propellinga drying gas through and/or over a) the material, b) a condenser of aheat pump and c) a variable heat exchange area of the heat pump whichevaporates refrigerant and which divides the drying gas into two or moresub streams which pass over at least some of the evaporator heatexchange area in a functionally parallel configuration and at least partof this evaporator heat exchange can be controlled to make it eithermore or less active for heat exchange as well as controlling both therefrigerant flow in the heat pump and the total active evaporator heatexchange area to assist in optimising the efficiency of drying thematerial.

More particularly the invention comprises a process for drying amaterial using a drying gas including:

causing a first sub-stream of the drying gas to flow through a heat sinkheat exchanger to cool and condense liquid out of the drying gas, with aheat transfer surface arranged to exchange heat with a first sub-streamof the drying gas, causing a second sub-stream of the gas to flowthrough a heat source heat exchanger to heat the drying gas, with a heattransfer surface arranged to exchange heat with said second sub-streamof the drying gas, said heat source heat exchanger being arranged in afunctionally parallel with said heat sink heat exchanger so that each ofsaid drying gas sub-streams exchanges heat with one of the two said heattransfer surfaces per cycle through the heat exchange system, andcausing the flow direction of the drying gas through the heat sink andheat source heat exchangers to reverse.

In another aspect the present invention may be said to consist in a heatpump including a working fluid circuit with a refrigerant, a means ofcompressing a variable flow of refrigerant, a condenser, variable heatexchange area which evaporates refrigerant and which has at least somearea in a functionally parallel configuration relative to the flow of aheat source medium and at least part of which can be controlled to makeit either more or less active for heat exchange, and a controller foroperating the means of compression and the evaporator heat exchange areain a manner to assist in optimising efficiency during operation.

In another aspect the present invention may be said to consist in a heatpump including: a working fluid circuit with a refrigerant, one or morecompressors in the circuit for compressing the refrigerant, a condenserin the circuit for exchanging heat between the refrigerant and a heatsink medium, variable evaporator heat exchange area in the circuit forexchanging heat between the refrigerant and a heat source medium andwhich has at least some area in a functionally parallel configurationrelative to the flow of a heat source medium and at least part of whichcan be controlled to make it either more or less active for heatexchange, and a controller for selectively increasing or decreasingcompressor functionality to control refrigerant flow rate through thecircuit and thus also the amount of heat moved by the heat pump betweenthe evaporator and the condenser and the power consumed by the heatpump, and for increasing or decreasing the active evaporator heatexchange area to control heat exchange between the refrigerant and theheat source medium.

Also, in reducing the active area for heat exchange at the evaporator,the fraction of heat source medium flow over that active heat exchangearea relative to the total heat source medium flow is reduced. Thus atleast some of the variable heat exchanger area would be configured in afunctionally parallel manner so that when the active area for heatexchange is reduced, the fraction of the total flow of the heat sourcemedium in heat exchange with the active evaporator area is also reduced.For example if the active evaporator heat exchange area is cut by somefraction, the heat source medium flow path would be left as it wasbefore the active area was reduced so that part of the heat sourcemedium flows over the remaining active area while the rest continues toflow over the inactive area in an effective bypass of the remainingactive area.

In another aspect the present invention may be said to consist in amethod of operating a heat pump for drying a material including: sensinga wet-bulb temperature and dry-bulb temperature in a drying gas flow, ina first drying stage after initial heat-up, controlling the rate of heatrejection from the drying gas flow to maintain the wet-bulb temperaturesubstantially constant and allow the dry-bulb temperature to rise toincrease the driving force for moisture removal and thus maintain therate of moisture removal from the system for a longer part of theprocess, and in a second drying stage when the dry-bulb temperaturereaches a limit, also controlling refrigerant flow through the heat pumpto vary the rise in or maintain the dry-bulb temperature and optionallyvary the wet bulb temperature to adjust the driving force for moistureremoval from the material being dried to control the quality of thematerial being dried.

In another aspect the present invention may be said to consist in anapparatus for drying a material including: a chamber for a material, aheat pump for drying the material using a drying gas flow, sensors fordetecting wet-bulb and dry-bulb temperatures of the drying gas flow, anda controller for controlling operation of the heat pump based onwet-bulb and dry-bulb temperatures, wherein the controller operates theheat pump to: a) in a first drying stage after initial heat-up, controlthe rate of heat rejection from the drying gas flow, to maintain thewet-bulb temperature substantially constant and allow the dry-bulbtemperature to rise to increase the driving force for moisture removaland thus maintain the rate of moisture removal from the system for alonger part of the process, and in a second drying stage when thedry-bulb temperature reaches a limit, control refrigerant flow throughthe heat pump to vary the rise in or maintain the dry-bulb temperatureand optionally vary the wet bulb temperature to adjust the driving forcefor moisture removal from the material being dried to control thequality of the material being dried.

Optionally, the moisture removal rate is also sensed to assist incontrolling the heat rejection rate and refrigerant flow through theheat pump to optimise drying.

The heat exchange apparatus may be a heat pump with an evaporator andcondenser as the hot and cold heat exchangers respectively.Alternatively, the heat exchange apparatus may utilise other integratedheat exchange technology. For example, other heat sinks and sources maybe used to augment or replace the heat pump evaporator and condenser.

Preferably, the invention provides the even drying benefits of atraditional reversing heat-and-vent method and system plus the energyefficiency and other related benefits of a heat pump or heat integratedmethod and system as well as the benefits of improved integrated controlof both the heat pump and drying process.

A preferred embodiment of the invention consists of a heat pump dryingprocess and apparatus configured so that the heat pump condenser andevaporator are located entirely within the kiln chamber and workeffectively with the primarily closed loop recirculating air-flow (orother drying gas medium) in either direction. This system is combinedwith the method and means to reverse that drying gas flow. The methodand apparatus of the invention conducts the drying gas cooling andmoisture condensation heat exchange at the heat pump evaporator and thedrying gas heating heat exchange at the heat pump condenser in aconfiguration functionally parallel to the drying gas flow rather thanin a sequential series configuration as is done with conventional heatpump dehumidifier drying systems. Thus the drying gas is split into twoor more sub streams in a functionally parallel configuration such thatat least one sub stream exchanges heat with only the heat pumpevaporator and at least one other sub stream exchanges heat with onlythe heat pump condenser.

Preferably, compressor functionality in the heat pump circuit(refrigerant flow) can be selectively increased or decreased by a clearmeans of control associated with the compressor system. Individualcompressors within the compression system may also be selectively shutoff or turned on as a means of controlling the refrigerant flow in theheat pump circuit. Controlling refrigerant flow controls the rate ofheat gain by the refrigerant from the drying gas through the evaporatorarea, thus controlling cooling of the drying gas. Controllingrefrigerant flow also controls the rate of heat transfer to the dryinggas by the refrigerant through the condenser, thus controlling heatingof the drying gas. Controlling the refrigerant flow also helps controlthe power consumed by the process so matching the refrigerant flow tothe needs of the drying system will improve the overall efficiency ofthe drying process.

Preferably, the evaporator variable heat exchange area can beselectively increased or decreased by operating refrigerant controlvalves associated with the evaporator areas, to activate and de-activatethem as required. Other control mechanisms could also be envisaged,however within the scope of this invention. In this manner, thevariation in heat exchange area is such that sections of heat exchangearea, in a functionally parallel configuration relative to the dryinggas medium, are put into and out of active heat exchange service withthe drying gas medium. The evaporator variable heat exchange area may beformed from one evaporator with multiple sections that can be activatedor de-activated as required, or multiple independent evaporators thatcan be independently activated or de-activated. Multiple independentevaporators may also each comprise multiple sections, each of which canbe activated or de-activated.

It is also preferable to configure this variable evaporator heatexchange area such that two or more of the sub streams of drying gaspass over separate sections of the evaporator heat exchange area. Theeffective evaporator heat exchange area is then adjusted according tothe specific drying gas flow configuration such that the drying gasflowing across the active evaporator area is always cooled sufficientlyto condense and remove liquid from the drying gas in combination withadjusting total refrigerant flow through the compression system whilekeeping the total effective heat pump condenser heat exchange area inthe drying gas stream constant. This will have the initial benefit ofkeeping the evaporating and condensing temperatures within the allowedranges for the compressor system. This will also have the benefit ofdriving the drying process at higher efficiency over a range of dryingconditions since it will enable the wasteful excess moisture removalcapacity of the heat pump system to be turned down, while keeping theevaporating temperature at the optimum value for efficient operation asthe material dries and inherently releases moisture more slowly. Anotherbenefit is realised by keeping the condenser fully active throughout thedrying process which mininises the condensing temperature difference sothe efficiency of the system is kept near its maximum as the drying gastemperature rises during the drying process. This enables the operatingtemperature to be increased to increase the driving force for drying,while remaining within the compressor operating limits, when theinherent drying rate naturally drops off later in the drying process.

Thus with this preferred embodiment, the performance of the drier can beoptimised during the start of the drying process at high heat pump loadswhen the temperature is lowest, and the humidity highest using a highrefrigerant flow in the heat pump and a large active evaporator area.Yet the present invention will still permit the drier to operateeffectively and efficiently at high temperatures and low humidity underlow heat pump loads, as required to complete the drying process as fastand efficiently as possible using a lower refrigerant flow, lower activeevaporator area, and higher active condenser area per unit refrigerantflow and also permit heat transfer to enable the higher dry bulbtemperatures for the drying gas flow to be achieved more efficiently andthe moisture from the drying gas stream to continue to condense and beremoved from the process. Furthermore, all of this is accomplishedwithout disrupting the drying gas flow or negatively affecting thepressure drop in the drying gas circuit.

In the preferred embodiment, the control of the heat rejected from thedrying process is based on the wet bulb temperature of the drying gassuch that the wet bulb temperature is kept nominally constant for anextended period during the drying process and the flow of refrigerant inthe heat pump is controlled based on the dry bulb temperature of thedrying gas such that the dry bulb temperature is kept within certainlimits throughout the drying process.

In optional embodiments, it is possible to use a waste heat source tosupplement or replace the heat pump condenser and a waste heat sink suchas cooling water to supplement or replace the heat pump evaporator. Itis also possible to run the drying gas in a more open loop configurationwhere part or all of the sub stream passing over the heat pumpevaporator or other cold exchanger is vented from the process aftertransferring and recycling heat back to the process through that heatpump evaporator or other cold exchanger while a fresh drying gas substream is introduced as make up to the process to replace that which isvented.

In the preferred embodiments of the invention, in each pass through theheat pump system, part of the drying gas passes over the heat pumpevaporator where some of the moisture is condensed out and part of thedrying gas passes over the heat pump condenser and is heated up. Byaccepted practice, this functionally parallel heat exchangerconfiguration in the present invention with two or more sub streamsseparately passing over the heat pump evaporator and condenser beforeremixing would not be expected to provide efficient or adequate netheating of the drying gas medium to drive the drying of the material inquestion in any reasonably economic way. This is because heat is bothremoved from and added to the sub-streams of the drying gas streambefore they are remixed and the effects of this functionally parallelheating and cooling would be lost in most design configurations.However, it has been unexpectedly found that through judicious design ofthe associated evaporator and condenser heat exchangers and the flow ofthe drying gas sub-stream components as described in the invention, thisconfiguration gives unexpectedly high energy and drying efficiency, veryclose to that achievable in a traditional series functional heatexchanger configuration while providing the increased quality benefitsassociated with a process where the drying gas flow is periodicallyreversed.

As with other existing heat pump systems, for low humidity operation,the drying capacity and efficiency of the invention can be optionallyenhanced by recovering sensible cooling at the evaporator using a pairof liquid coupled or heat-pipe coupled heat exchangers at the evaporator(Blundell, 1979).

As those skilled in the art will appreciate, the process and apparatusof this invention will provide benefits to drying many differentmaterials. These materials include but are not limited to timber,boards, paper, bricks, milk, gypsum, plaster board, textiles, chinaclay, fertilizer, dye stuffs, tiles, pottery, grain, nuts, seeds,fruits, bio-processing waste, etc.

The process and apparatus of this invention are also amenable to variousdrying gas mediums. Although the preferred embodiment for the inventionis with air as the drying gas, the process and apparatus can beconfigured to use O2-free air, nitrogen, argon, oxygen, or any othergaseous medium to take up the moisture from the materials to be driedand condense that moisture out of the system through the heat pumpevaporator as noted in (Chen, Bannister, McHugh, Carrington, Sun, 2000)for other more traditional heat pump drying systems. As with otherexisting heat pump systems, the invention requires means for rejectingexcess heat from the kiln chamber. This may include full time orperiodic venting of the drying gas, cooling the drying gas entering theevaporator, cooling any make-up or purge drying gas entering or leavingthe apparatus, sub-cooling the liquid heat pump refrigerant leaving thecondenser, cooling the heat pump refrigerant leaving the compressor, orcooling and partially or wholly condensing the high-pressure refrigerantfor purposes of control.

Also, although the system is preferentially focussed on water removal,it can also be configured to remove other vaporisable and condensableliquids from the material to be dried such as various organic solventsto be recovered from solvent based processing steps including painting.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be described with referenceto the accompanying drawings, of which:

FIG. 1 shows a basic heat pump process flow diagram,

FIGS. 2A and B show preferred heat exchanger and drying chamberconfigurations in forward and reverse drying gas flow,

FIGS. 3A and B show the detail of the overall heat exchangeconfiguration in forward and reverse drying gas flow,

FIG. 4 shows a heat pump process flow diagram with separate evaporatorsections and multiple compression devices each arranged in functionallyparallel configurations independent of airflow direction,

FIG. 5 shows a heat pump drying system with nominal flow in the forwarddirection with both a preferred overall heat pump condenser andevaporator configuration and a preferred variable evaporator areaconfiguration,

FIG. 6 shows an example temperature profile during timber drying, and

FIG. 7 shows a graph comparing drying performance with respect toevenness of drying.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a process and apparatus to improve the heatpump based or heat integrated drying of timber and other materials. Apreferred embodiment of the invention involves conducting the heatingand cooling/partial condensing of two sub-streams of drying gas flow byindirect heat exchange against the respective heat pump condenser andevaporator in functionally parallel sub-stream flow paths such that atleast one sub stream exchanges heat with substantially only the heatpump evaporator and at least one other sub stream exchanges heat withsubstantially only the heat pump condenser with the ability toefficiently reverse the direction of drying gas flow through thecorresponding heat exchangers. Another preferred embodiment of theinvention involves conducting the cooling/partial condensing of one ormore sub streams of drying gas flow by indirect heat exchange againstthe heat pump evaporator configured such that each of these sub streamsis in a functionally parallel configuration and passes over differentareas of the heat pump evaporator that may be active or inactive forheat exchange. The overall flow of refrigerant through the heat pumpsystem is then controlled along with the active area for heat exchangein the evaporator to increase efficiency. In another preferredembodiment, this control along with control of the rate of heat rejectedfrom the overall drying system is then preferably provided based onsensing the wet bulb and dry bulb temperatures of the drying gas streamsuch that the heat rejection rate is varied to keep the wet bulbtemperature nominally constant for an extended period during the dryingprocess while the flow of refrigerant through the heat pump and theactive evaporator heat exchange area are varied to keep the dry bulbtemperature within certain limits.

The following description of the process and apparatus of thisinvention, by way of example only and with reference to the accompanyingdrawings in the accompanying figures, indicates the presently preferredembodiments of the invention.

Referring to FIG. 1, the basic heat pump cycle is put forward with theprimary sequence of processes for the refrigerant cycle of compression11, condensation 12, expansion 13 and evaporation 14 with the drain 15to indicate the removal of condensed liquid from the drying gas stream(not shown) at the evaporator 14 and stream 16 retuning to thecompressor to indicate the closed loop nature of the refrigerant flow.

In the context of a dehumidifier drying system 20, referring to FIG. 2A,the heat pump compressor 25 operates to move heat from the lowertemperature evaporator heat exchanger 27 to the higher temperaturecondenser heat exchanger 26. With these heat pump evaporator 27 andcondenser 26 heat exchangers in a parallel configuration in the dryinggas path 29, the part of the drying gas 29A passing over the evaporatorheat exchanger 27 will lose heat, decrease in temperature and some ofthe moisture carried by that drying gas stream will condense while thepart 29B of the drying gas passing over the condenser 26 heat exchangerwill take up heat and increase in temperature. It is important to notethat the functionally parallel configuration refers to the relationshipbetween the heat exchangers 26, 27 and the drying gas flow 29, that is,the heat exchangers 26, 27 are arranged in functionally parallelconfiguration such that sub streams of the drying gas flow exchange heatsubstantially either with exchanger 26 or with exchanger 27. The termdoes not refer to the physical main geometric axis of the heatexchangers 26, 27 being in parallel with respect to each other. Heatpump operating parameters will be set such that the part of the dryinggas that cools down in passing over the heat pump evaporator 27 heatexchanger will drop to a temperature below its dew point and some liquidwill condense out of the vapour phase and be caught in drain 34 to beremoved from the system.

After the drying gas has passed over the heat pump evaporator 27 orcondenser 26 heat exchangers, each part of the drying gas 29A, 29B hasan increased capacity to take up moisture. Upon mixing these twosub-streams (in this case with the fan system 35), the combined dryinggas 29 will also have an increased capacity to take up moisture and thusprovide the unexpectedly high efficiency of the overall drying processfor this alternative configuration.

Considering the situation in which the drying gas flow 29 is in theanti-clockwise direction through the system in FIG. 2A, the two dryinggas sub-streams 29A, 29B then pass through a reversible fan system 35(or other mechanism for creating a drying gas flow) which provides themotive force to circulate the drying gas 29 through the overall systemand acts to mix the two sub-streams 29A, 29B into a single nominallyhomogeneous drying gas stream 29. In the anti-clockwise flow direction,the single drying gas stream 29 is guided through the systemsuperstructure 20 in the section of the superstructure 22 by variousflow conditioning devices 30 which act to minimise pressure drop in thesystem. An additional device 31 is shown to guide the drying gas flow 29around the system and through the material to be dried 23 in a singlepass configuration. It should be apparent to those skilled in the artthat this drying gas flow guide 31 could be configured in many variousways to achieve different paths for the drying gas 29 to flow throughthe material to be dried 23.

Once the drying gas has passed over and/or through the material 23 to bedried and picked up moisture evaporating from the material, it returnsto the heat pump though partition 19 and continues to recirculatethrough the system. It can be appreciated by those skilled in the artthat the drying gas flow need not be recirculated in a rigorously closedloop. It is readily possible within the scope of the invention to havevarious drying gas purge and makeup streams as is appropriate to thespecific drying application.

In the reverse flow configuration shown in FIG. 2B, the process isfundamentally the same except the sequence of the gas flow cycle 28proceeds in the clockwise direction. Reversal of airflow takes place ata suitable interval known to those skilled in the art to achieve evendrying. (Keey, Langrish and Walker, 2000) Starting with the heat pumpevaporator 27 and condenser 26 heat exchangers, the sub-streams 28A, 28Bof drying gas passing over each respective exchanger 27 and 26, wouldthen both pass through partition 19 which optionally could be configuredto act as a mixing device to homogenise the two drying gas sub streams28A, 28B. It is also possible to allow the turbulent flow of the gas toprovide mixing of the drying gas sub streams as part of their flowthrough the process. As in the anti-clockwise case, the drying gas wouldhave an increased capacity to take up moisture as it next passes overand/or through the material 23 to be dried where it takes up somemoisture evaporating from the material. After passing over and/orthrough the material 23 to be dried, the drying gas then passes aroundthe inside of the superstructure 20 in the section 22, aided by the flowconditioning devices and guides 31 and 30 before entering the reversiblefan system 35. Upon exiting the reversible fan system 35, the drying gasthen passes over the heat pump evaporator 27 and condenser 26 heatexchangers as two parallel sub streams 28A, 28B completing the clockwisecycle of flow and moisture removal in a functionally equivalent way tothe anti-clockwise cycle of flow.

It can be appreciated by those skilled in the art that other waste heatsources or sinks may be available at low cost in certain processenvironments. In these situations, for the case where the heat pumpsystem is augmented or replaced by an alternate high temperature heatsource and lower temperature heat sink, the condensing duty from theheat pump refrigerant working fluid is augmented or replaced by the hightemperature heat source in the heat exchange system and the evaporatingduty from the heat pump refrigerant working fluid is augmented orreplaced by the lower temperature heat sink in the heat exchange system.

The detail of heat exchange configuration in forward and reverse dryinggas flow shown in FIGS. 3A and 3B indicates how the drying gas contactsand transfers heat with the heat pump evaporator and condenser heatexchangers in a functionally parallel-gas-flow configuration. Thisfunctionally parallel-gas-flow configuration is best explained in thecontext of dividing the drying gas flow into two or more sub-streams28A, 28B, 29A, 29B which exchange heat with either the condenser 26(sub-stream A), the evaporator 27 (sub-stream B), or as is shown in theFIGS. 3A and 3B, with neither the evaporator nor the condenser as anoptional bypass sub-stream C. It can be appreciated by those skilled inthe art that a small sub-stream of drying gas could indeed exchange heatwith both the evaporator 27 and the condenser 26 as an additionaloptional configuration not shown. In addition, the specific geometry ofthe exchangers shown can be altered through various angles, rotations ordislocations without materially changing the invention. The separationof the drying gas into these sub stream will result from thefunctionally parallel configuration of the heat exchangers 26 and 27while the remixing will result from the gas moving device in onedirection and either the natural turbulence of its flow or through theaddition of an auxiliary device depending on the particular requirementsfor even mixing of the gas sub streams.

As described earlier in the context of FIGS. 2A and B when other wasteheat sources or sinks are available at low cost in certain processenvironments, it is readily possible to use such high temperature heatsources to supplement or replace the condensing duty in heat exchanger Cin FIGS. 3A and 3B. Similarly in such cases, it is readily possible touse such lower temperature heat sinks to supplement or replace theevaporating duty in heat exchanger E in FIGS. 3A and 3B

FIG. 4 is a simplified process flow diagram which illustrates furtherimprovements which may be applied to a heat pump drier. Here the heatpump refrigerant is compressed from low pressure by a compression systemdesignated by separate compressor modules 101, 102 and 103. The flow ofrefrigerant is controlled to each of these compressor modules by valves104, 105 and 106 respectively. The control signal for these valves comesfrom an integrated control system 117 which in turn takes input from oneor more sensors 116 in the refrigerant stream, the drying gas medium,the material being dried, and/or the moisture extracted from the dryingmaterial. A preferred embodiment of the invention specifically focuseson wet bulb and dry bulb temperature sensors in the drying gas streamalthough other options are not excluded. When any of the control valves104, 105 or 106 are shut completely, the integrated control system willalso signal the corresponding compressor module or modules to shut offto save power and improve the process efficiency. This configurationillustrates the example of control of the total refrigerant flow in theheat pump using a multi-compressor compression system and associatedsuction valves. This compression and refrigerant flow control will alsocorrespondingly control the rate of heat movement between the evaporatorand condenser of the heat pump system as well as the total powerconsumption by the heat pump system. It is also acknowledged thatvarious other compression systems, such as a positive displacementcompressor run by a variable speed drive, can also accomplish thisefficient control of total refrigerant flow in the heat pump circuit aspart of the present invention.

The high pressure refrigerant is then condensed in heat exchanger 107which provides at least part of this heat of condensation to the dryinggas stream. The condensed high pressure refrigerant then passes to twoor more parallel evaporator heat exchange areas 112, 113, 114 and 115through their respective expansion control valves 108, 109, 110 and 111.This arrangement results in a variable heat exchange area forevaporating refrigerant, sections of which can be activated andde-activated as required to alter the effective heat exchange area. Inthis manner, the variation in heat exchange area is such that sectionsof heat exchange area, in a functionally parallel configuration to eachother relative to the drying gas medium, are put into and out of activeheat exchange service with the drying gas medium. Note that the numberof evaporation modules need not equal the number of compression modulesnor is it required for there to be a one to one correspondence with saidcompressor modules although the general mode of operation will be suchthat the active evaporator area is typically decreased as therefrigerant flow through the overall heat pump circuit is reduced. Itwill be appreciated that the variable evaporator heat exchange totalarea could be constructed in a various ways. For example, the evaporatorvariable heat exchange area may be formed from one evaporator withmultiple sections that can be activated or de-activated as required, orwith multiple independent evaporators that can be independentlyactivated or de-activated. Multiple independent evaporators may alsoeach comprise multiple sections, each of which can be activated orde-activated.

As for the compression system and the total flow of refrigerant in theheat pump circuit, the refrigerant flow through each of these evaporatorheat exchange areas is controlled through integrated control system 117which in turn takes input from the drying gas wet bulb and dry bulbtemperature sensors 116. Although it is noted that other sensors in therefrigerant stream, the drying gas medium, the material being dried,and/or the moisture extracted from the drying material may optional beused. Each of these evaporator heat exchange areas is specificallypositioned in a functionally parallel configuration to each otherrelative to the drying gas flow, described later in the context of FIG.5, to remove at least some heat from the drying gas stream such thatmoisture from that drying gas stream is condensed out and removed fromthat drying gas stream. After passing through the evaporator heatexchanger system, the low pressure evaporated refrigerant returns to thecompressor system in a standard recirculation flow configuration.

Referring to the simplified heat pump drier configuration diagram inFIG. 5 with the drying gas flowing through the drier in ananti-clockwise direction, the heat pump compressor system 201 operatesto move heat from the lower temperature parallel evaporator heatexchangers 202A, B, and C to the higher temperature condenser heatexchanger 203. Any excess heat is removed from the drying gas forcontrol purposes through exchanger 216. Although it is shown upstream ofthe heat pump condenser and evaporator in the anti-clockwise flowdirection in this figure, it will be understood by those skilled in theart that there are numerous other locations possible for an exchanger toremove such excess heat for control purposes.

As the drying gas passes over the evaporator heat exchangers it willlose heat and decrease in temperature and as the drying gas passes overthe condenser heat exchanger it will take up heat and increase intemperature. Heat pump integrated control parameters will be set suchthat as the drying gas that cools down in passing over the heat pumpevaporator heat exchangers it will drop to a temperature below its dewpoint and some liquid will condense out of the vapour phase and becaught in drain 204 to be removed from the system. It is important tonote that the evaporator heat exchange area under control isspecifically configured such that when only part of the evaporator heatexchange area is active, the drying gas flow will continue over theinactive heat exchange area without coming into thermal contact with theactive area. This specifically allows the heat removal by the evaporatorto be concentrated over a fraction of the drying gas flow so thatsufficient liquid will condense from that part of the drying gas streamto continue the drying process. This configuration can be considered asdividing the drying gas flow into two or more sub streams which theneither pass over the heat pump condenser heat exchanger and are heatedor pass over the heat pump evaporator exchanger and are cooled and themoisture carried by that sub stream is partially condensed and drainedfrom the system.

The part of the drying gas that passes over the heat pump condenserexchanger 203 where is heated and combined with the part of the dryinggas that passed over the heat pump evaporator and with any other substreams of drying gas that may have been optionally split out beforepassing through a fan system 205 which provides the motive force tocirculate the drying gas through the overall system. The drying gasstream is then guided through the system superstructure 211 in thesection of the superstructure 206 by various flow conditioning devices210 which act to minimise pressure drop in the system. An additionaldevice 207 is shown to guide the drying gas flow around the system andthrough the material to be dried 208 in a single pass configuration. Itshould be apparent to those skilled in the art that this drying gas flowguide 207 could be configured in many various ways to achieve differentpaths for the drying gas to flow through the material to be dried 208.

Once the drying gas has passed over and/or through the material to bedried 208 and picked up moisture evaporating from the material, itreturns to the heat pump though partition 209. The evaporatorconfiguration, the corresponding refrigerant flow control and the dryinggas flow arrangement may or may not be combined with reverse flowcapabilities depending on the requirements and limitations of aparticular application.

Although there are many possible ways to provide control for thisprocess and apparatus, a preferred embodiment of the invention is tocontrol the heat pump and the drying process in concert throughrejecting heat from the process using input from the drying gas dry bulband wet bulb temperature sensors and optionally the amount of totalliquid removed from the system such that the wet bulb temperature iskept constant through the main drying period while the dry bulbtemperature increases to provide the optimum driving force for moistureextraction from the material being dried as measured by the amount ofliquid removed from the system through the drain line or the differencebetween the wet and dry bulb temperatures within the limits of the heatpump system capabilities. Then when the drying process has progressed tothe point where the drying gas dry bulb temperature reaches a maximumvalue based on the limits of the heat pump compressor system, thecontrol adjusts the total refrigerant flow through the compressionsystem down while keeping the drying gas wet bulb temperature largelyconstant.

The specific hierarchy of control in this preferred embodiment initiallyruns the process at the maximum drying capacity and rate of heatrejection. To maintain the overall stability of the process and heatpump operation at the highest drying rate and most efficient heat pumpconditions, the preferred embodiment then increases the dry bulbtemperature as the drying progresses while maintaining the wet bulbtemperature roughly constant. Then when the dry bulb temperature reachesa predetermined maximum, the heat pump refrigerant flow is reduced tolimit the further rise in dry bulb temperature and reduce the powerconsumption of the heat pump. As this maximum is approached, the wetbulb temperature may then optionally be varied to limit the overalldriving force for drying the material to prevent internal stresses fromdamaging the material being dried based on a combination of thedifference between the wet and dry bulb temperatures and the rate ofoverall moisture extraction from the system.

This new control scheme has the benefit of keeping the evaporating andcondensing temperatures within the allowed ranges for the heat pumpcompressor system as well as driving the heat pump system and the dryingprocess at their maximum efficient states according to the naturaldrying rate reduction as the drying process progresses This comes out ofincreasing the drying gas dry bulb temperature at a substantiallyconstant rate of dehumidification in order to increase the driving forcefor drying, while maintaining a slower variation in the wet bulbtemperature over the length of the drying process to smoothly remainwithin the compressor operating limits and properly manage the stressespresent in the material being dried, as the inherent drying ratenaturally drops off as the drying process progresses. Thus in responseto the falling drying rate of the material being dried, the new controlsystem automatically increases the drying force applied to the productin order to substantially maintain the drying rate. Because theadjustment in the drying force can be linked to the moisture content ofthe product, the driving force can be controlled to ensure it isconsistent with the capacity of the material to tolerate theprogressively more aggressive drying conditions. The result is that themaximum drying rate is maintained longer than with the prior art, andthe drying end-point is achieved more quickly while avoiding dryingconditions that could damage the product.

It is important to note that although the temperature sensors have beenshown in FIG. 5 where the drying gas enters the material drying chamber,there are numerous other functionally equivalent locations where thetemperature sensors can be located in the drying gas flow stream withoutmaterially changing the invention. Furthermore, additional systemprotection sensors can also be included in the heat pump refrigerationcircuit without materially changing the invention but they would notprovide primary operational control for the process in the preferredembodiment.

Thus, the performance of the drier can be optimised during the start ofthe drying process to ensure the heat pump is highly loaded when the drybulb temperature is lowest, and the humidity highest using a highrefrigerant flow in the heat pump. The preferred embodiment will alsocontrol the drier to maintain the maximum possible drying rate as longas possible. Then, when it is no longer possible to maintain the maximumdrying rate because of drying material stress and transport limitations,the control will manage the heat pump so that it operates effectivelyand efficiently at higher dry bulb temperatures and lower humidity underlower loads, as required to complete the drying process as fast andefficiently as possible using a lower refrigerant flow and higher activecondenser area per unit refrigerant flow. This control will alsomaximise heat transfer at the condenser to enable the higher dry bulbtemperatures for the drying gas flow to be achieved more efficiently.Furthermore, all of this is accomplished without disrupting the dryinggas flow or negatively affecting the pressure drop in the drying gascircuit.

It can also be appreciated by those skilled in the art, that additionalcomponents specific to the product being dried, such as auxiliaryheaters for sterilization, and water spray systems for reconditioningcan be readily added to the process and apparatus of the inventionwithout materially changing the invention.

Similarly there are various methods and apparatus that can be added tothe process and apparatus of this invention to reject excess heat fromthe overall process to the ambient environment without materiallychanging the invention as is shown for example only by item 216 in FIG.5. These include but are not limited to venting a sub-stream of dryinggas, pre-cooling the drying gas entering the evaporator, cooling anymake-up or purge drying gas entering or leaving the heat pump apparatus,sub-cooling the liquid heat pump refrigerant, de-superheating the heatpump refrigerant leaving the compressor, or partially or whollycondensing the high-pressure refrigerant for purposes of control.

As with other heat pump systems, additional methods of heat recovery maybe optionally applied to the invention without material change to theinvention. For instance, it is possible to include the capacity forreclaiming sensible cooling at the evaporator using, for example, eithera pair of liquid coupled heat exchangers, or by means of heat-pipecoupled heat exchangers.

Also, it is within the scope of this invention to include auxiliary heatsources and sinks separate from the heat pump circuit to enhance andaugment the heating of the drying gas by the heat pump condenser and thecooling and partial condensation of the drying gas by the heat pumpevaporator without materially altering the invention itself.

As those skilled in the art will appreciate, the process and apparatusof this invention will provide benefits to drying many differentmaterials. These materials include but are not limited to timber,boards, paper, bricks, milk, gypsum, plaster board, textiles, chinaclay, fertilizer, dye stuffs, tiles, pottery, grain, nuts, seeds,fruits, bio-processing waste, etc.

The process and apparatus of this invention are also amenable to variousdrying gas mediums. Although the preferred embodiment for the inventionis with air as the drying gas, the process and apparatus can beconfigured to use O2-free air, nitrogen, argon, oxygen, or any othergaseous medium to take up the moisture from the materials to be driedand condense that moisture out of the system through the heat pumpevaporator. As with other existing heat pump systems, the inventionrequires means for rejecting excess heat from the kiln chamber. This mayinclude desuperheating, condensing or sub-cooling refrigerant leavingthe compressor and rejecting heat to the environment. Alternatively thedrying gas may be precooled as it enters the evaporator or thedehumidifier more generally.

Also, although the system is preferentially focussed on water removal,it can also be configured to remove other vaporisable and condensableliquids from the material to be dried such as various organic solventsto be recovered from solvent based processing steps including painting.

Although the Figures show preferred embodiments for timber processing,it can readily be appreciated that minor changes to the drying chamberconfiguration can be made to facilitate the drying of other materials,in other drying gas mediums and for removing liquids other than water.

In the preferred embodiments for timber drying for a typical charge ofgreen timber with 150% moisture content to start and drying to a 10%moisture content before any optional spray reconditioning, the nominalconditions are summarised in Table 1: TABLE 1 Timber Drying ExampleParameter Range Dry bulb temperature of drying gas 35-70 C. (averageover the system) Wet bulb temperature of drying gas 20-65 C. (averageover the system) Drying gas velocity through drying product 2-5 m/sApproach temperature in heat pump condenser 2-25 C. Approach temperaturein heat pump evaporator 2-45 C. Drying gas temperature rise acrosscondenser heat 3-15 C. exchanger Drying gas temperature drop acrossevaporator heat 3-35 C. exchanger Condenser temperature heat pump fluidside 40-85 C. Evaporator temperature heat pump fluid side 20-65 C.

For lumber drying in conventional heat-and-vent kilns, air flow reversalreduces the variation in the moisture content of boards along thedirection of air flow within the timber stack (Keey, Langrish, Walker,2000). An example of how the current invention achieves this benefitthis is shown in FIG. 7 which illustrates how the maximum difference inthe moisture content of different boards in a stack of Pinus radiatawould be expected to vary with time for a dehumidifier kiln based ondetailed computational modelling. In this calculation example, reversalsare carried out at intervals of 12 hours. Comparison of the two curvesin the figure shows that the variation in moisture content is smallerwhen air flow is reversed in this way than when the air flow isunidirectional. This also shows that a given target range of moisturecontent can be achieved more quickly under an air flow reversal regimethan under unidirectional air flow. Thus the reverse flow aspect of theinvention achieves a performance benefit relative to the prior art heatpump drying systems.

The performance of the variable active evaporator and correspondingrefrigerant flow control aspect of this invention is also expected to besuperior to the existing technology based on the following arguments.With a conventional monolithic evaporator the compressor capacity can beturned down when the drying rate of the product falls, but there is adanger that the evaporating temperature of the refrigerant will exceedthe allowed limits, since the evaporator is oversized relative to thelow refrigerant flow and the refrigerant will be heated to a highertemperature in the exchanger. Consequently, it is necessary to ensurethat the refrigerant temperature does not exceed preset limits underthese conditions which will limit how far the compressor can be turneddown to improve the drying efficiency during the later parts of thedrying process. The alternative with the existing technology is to cyclethe heat pump on/off when the drying rate falls. This is unsatisfactory,since it shortens the operating life of the heat pump. In addition, thedrying gas temperature is still limited by the normal limits for theevaporating and condensing temperatures. In timber drying this meansthat the finishing stages of the drying process normally must becompleted at temperatures of 50-55° C. which limits the driving forcefor moisture removal from the timber being dried.

With the present invention considering the case of a system nominallycomparable to the existing technology described in the previousparagraph using three stages of capacity reduction at the compressor andevaporator segments of the heat pump cycle and a constant condenser heatexchange area, detailed computer simulation has shown that the dryinggas temperature can be increased progressively to 60-65° C. as thedrying rate naturally falls. This gives a higher driving force formoisture removal from the timber during the later phase of the dryingprocess, as illustrated in FIG. 6. As a consequence of this higherdriving force for moisture removal, (60-65° C. vs. the 50-55° C. of theexisting technology) the final stages of the drying process can besignificantly accelerated, reducing the drying time by typically 20%-30%according to these detailed computer simulations.

The performance of the control component of invention is expected to besuperior to the existing technology based on the following arguments.The proposed control strategy allows the dehumidifier to operate at itsmaximum potential capacity throughout the entire drying cycle by usingthe wet-bulb temperature as the primary measured variable forcontrolling the rate of heat rejection. Normally with existing controlsystems, the rate of heat rejection in heat pump drying kilns iscontrolled to maintain a given dry-bulb temperature. However with thatform of control the dehumidifier capacity undergoes large changes incapacity as the relative humidity varies during the drying process. Fordehumidifier dryers the drying capacity typically increases by 7% for 1°C. increase in the wet-bulb temperature with a fixed dry-bulbtemperature. It decreases by less than 3% for 1° C. increase in dry-bulbtemperature, at a fixed wet-bulb temperature.

In effect the system is approximately 2.3 times more sensitive to thewet-bulb setting than the dry-bulb. This is why it makes more sense toadjust the heat rejection to maintain the wet-bulb rather than thedry-bulb, provided both are within acceptable limits.

When the rate of heat rejection is controlled by measuring andmaintaining the wet-bulb temperature, the dry-bulb temperature rises asthe product dries. This is normally acceptable for the product, and isconsistent with many accepted heat-and-vent drying schedules. In effectthe natural drying trajectory of the heat pump dehumidifier kiln systemwhich uses the wet bulb as the primary measured control variable isalready close to accepted schedules, and this proposed invention makesuse of this feature. Eventually the dry bulb temperature will reach thesafe limit for the product being dried, or the dehumidifier will reachthe normal operating limits for the condensing temperature of thecompressors. As the limit temperatures are reached as indicated by thedrying gas dry bulb temperatures with optional confirmation from therefrigeration circuit sensors, the heat pump refrigerant flow is reducedby reducing the compressor capacity. Rather than using the measuredrefrigerant pressure to specify the limit conditions, this scheme usesthe dry-bulb temperature as an indicator. This is cheaper to do, and itintegrates well with the overall drying cycle control.

A preferred example of the function of the control system based ondetailed computer process simulation is shown in FIG. 6. The wet and drybulb temperatures start at an ambient of roughly 12° C. as read from theleft side of the graph and the drying run begins with a “Heat Up” phase.It is acknowledged that additional auxiliary heaters may be used toaccelerate this phase without materially affecting the invention. Whenthe system reaches the preset wet bulb temperature, in this case 41° C.,the control system acts to run the heat pump to extract the maximum rateof moisture removal and run the heat rejection coil to maintain that wetbulb temperature through adjusting the amount of heat rejected from thesystem. Detailed computer simulation has shown that the drying gastemperature can be increased progressively to 60-65° C. as the dryingrate naturally falls within the limits of commonly available compressorsas shown in the “Constant Wet Bulb with Increasing Dry Bulb” phase inFIG. 6. This gives a higher driving force for moisture removal from thetimber during the later parts of the drying process, as indicated by thedifference between the wet and dry bulb also illustrated in FIG. 6. Thisis also reflected in the ability of the control system to keep themoisture extraction rate closer to its maximum value for longer asmeasured on the right side of the graph. In cases where the drivingforce must be moderated because of limits present in the material beingdried, the wet bulb temperature can be increased to control the drivingforce within these material based limits as shown in the “Increasing WetBulb” phase in FIG. 6.

It will be appreciated that the invention is not restricted to theparticular embodiments and modifications described above and thatnumerous modifications and variations can be made without departing fromthe scope of the invention.

REFERENCES CITED

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1. A heat exchange system for a drying apparatus, including: a heat sinkheat exchanger to cool and condense liquid out of a drying gas, with aheat transfer surface arranged to exchange heat with a first sub-streamof the drying gas, and a heat source heat exchanger to heat the dryinggas, with a heat transfer surface arranged to exchange heat with asecond sub-stream of the drying gas, and arranged in a functionallyparallel configuration with said heat sink heat exchanger so that eachof said drying gas sub-streams exchanges heat with one of the two saidheat transfer surfaces per cycle through the heat exchange system, and agas movement device for propelling the drying gas through or around theheat sink and heat source heat exchangers in either a forward or areverse flow path direction.
 2. A heat exchange system according toclaim 1 where at least part of the heat source heat exchanger is acondenser in a heat pump system.
 3. A heat exchange system according toclaim 2 where at least part of the heat sink heat exchanger is anevaporator in the heat pump system.
 4. A heat exchange system accordingto any one of claims 1 to 3 arranged to heat the drying gas to atemperature between 25 and 90C.
 5. A heat exchange system according toclaim 3 wherein said gas movement device is a fan.
 6. A heat exchangesystem according to any one of claims 1 to 5 including a gas flow patharranged to substantially mix the two gas streams after they have passedthrough or around said heat sink and heat source heat exchangers.
 7. Aheat exchange system according to any one of claims 1 to 6 including acontrol system arranged to reverse the drying gas flow direction basedon any one or more of drying time, moisture content, wet or dry bulbtemperature or relative humidity of the drying gas, or integrated amountof moisture removed from the drying gas.
 8. A heat exchange systemaccording to claim 7 including sensor(s) for determining the dry-bulband wet-bulb temperatures and/or relative humidity for the drying gasflow entering and/or leaving the dehumidifier.
 9. A heat exchange systemaccording to either one of claims 7 and 8 wherein said control system isarranged to be able to reduce power consumption of the heat exchangersystem during drying by reducing the capacity of the heat sink heatexchanger to condense moisture out of the drying gas and/or the capacityof the heat source exchanger to heat the drying gas.
 10. A heat exchangesystem according to any one of claims 1 to 9 including means forrejecting heat from the drying apparatus to the external environment 11.A heat sink exchange system according to any one of claims 1 to 10arranged so that the drying gas passes over a substantially closed looppath repeatedly through the heat exchange system and past or through adrying chamber for containing a material to be dried.
 12. A dryingapparatus including: a chamber for material to be dried, a gas movementdevice for propelling a drying gas in alternative forward and reverseflow path directions through the chamber, a heat exchanger to cool andcondense liquid out of the drying gas, with a heat transfer surface toexchange heat with a first sub-stream of the drying gas, and a heatexchanger to heat the drying gas, with a heat transfer surface toexchange heat with a second sub-stream of the drying gas, said heattransfer surfaces being arranged so that the said drying gas sub-streamswill exchange heat with at most one of the two said heat transfersurfaces per cycle through the said heat exchangers.
 13. A heat pump fora drying apparatus including: a gas movement device for propelling adrying gas in alternative reverse flow path directions, an evaporator tocool and condense liquid out of the drying gas, with a heat transfersurface arranged to exchange heat with a first sub-stream of the dryinggas, and a condenser to heat the drying gas, with a heat transfersurface arranged to exchange heat with a second sub-stream of the dryinggas, and, such that at least part but less than a majority of the flowin the said drying gas sub-streams will exchange heat with both theevaporator and condenser in each cycle through the heat pump.
 14. A heatexchange apparatus operable in a drying apparatus with reversible dryinggas flow including a cold heat exchanger and a hot heat exchangerarranged such that during operation the heat exchangers lie in afunctionally parallel configuration relative to the drying gas flow,whereby a first sub-stream of the drying gas flow substantiallyexchanges heat with only the cold heat exchanger, and a second substream of the drying gas flow substantially exchanges heat with only thehot heat exchanger.
 15. A heat exchange system according to any one ofclaims 1 to 14 wherein said heat sink heat exchanger contains a heatsink medium to cool and condense liquid out of the drying gas, with aheat sink heat transfer surface comprising two or more sectionsconnected in a functionally parallel configuration with each otherarranged to exchange heat with two or more substreams of the drying gasso that each drying gas sub-stream exchanges heat with no more than oneof the two or more said heat sink heat transfer surface sections percycle through the heat exchange system.
 16. A heat exchange systemaccording to claim 15 with a control system arranged to control the flowof heat sink medium in the heat sink heat exchanger sections andincrease, decrease, turn on, and/or turn off the flow of heat exchangemedium independently in each of the heat sink heat exchanger sections.17. A heat exchange system for a drying apparatus including: a dryinggas to remove moisture from the material being dried, and a heat sourceheat exchanger containing a heat source medium to heat the drying gas,and a heat sink heat exchanger containing a heat sink medium to cool andcondense liquid out of a drying gas, with a heat sink heat transfersurface comprising two or more sections connected in a functionallyparallel configuration with each other arranged to exchange heat withtwo or more substreams of the drying gas so that each drying gassub-stream exchanges heat with no more than one of the two or more saidheat sink heat transfer surface sections per cycle through the heatexchange system.
 18. A heat exchange system according to claim 17 with acontrol system to control the flow of heat sink medium in the heat sinkheat exchanger sections and increase, decrease, turn on, and/or turn offthe flow of heat exchange medium independently in each of the heat sinkheat exchanger sections.
 19. A heat exchange system according to claim17 or 18 where at least part of the heat sink heat exchanger is anevaporator in a heat pump system.
 20. A heat exchange system accordingto claim 19 where at least part of the heat source heat exchanger is acondenser in a heat pump system.
 21. A heat exchange system according toany one of claims 17 to 20 arranged to heat the drying gas to atemperature between 25 and 90C.
 22. A heat exchange system according toany one of claims 17 to 21 including a drying gas flow path arranged tosubstantially mix the two or more of the said drying gas sub streamsafter they have passed through or around said heat sink heat exchangersections.
 23. A heat exchange system according to any one of claims 17to 22 including sensor(s) for determining the dry-bulb and wet-bulbtemperatures, relative humidity for the drying gas flow entering and/orleaving the dehumidifier, drying time and/or other indicator orindicators such as heat sink or source fluid temperature or pressure,moisture content of the material being dried, drying rate, or integratedamount of moisture removed from the system.
 24. A heat exchange systemaccording to any one of claims 17 to 23 wherein said control system isarranged to be able to temporarily reduce during drying, the overallcapacity of the heat sink heat exchanger to condense moisture out of thedrying gas and the overall capacity of the heat source exchanger to heatthe drying gas.
 25. A heat exchange system according to any one ofclaims 17 to 24 including means for rejecting heat from the dryingapparatus to the external environment such as full time or periodicdrying gas venting, pre-cooling the drying gas entering the evaporator,pre-cooling any make-up or purge drying gas entering or leaving theapparatus, sub-cooling the liquid heat pump refrigerant after it leavesthe condenser and before it enters the evaporator, de-superheating theheat pump refrigerant leaving the compressor, or partially or whollycondensing the high-pressure refrigerant for purposes of control.
 26. Aheat sink exchange system according to any one of claims 17 to 25arranged so that the drying gas passes over a substantially closed looppath repeatedly through the heat exchange system and past or through adrying chamber for containing a material to be dried.
 27. A dryingapparatus including: a chamber for material to be dried, a drying gas toremove moisture from the material being dried, and a heat source heatexchanger containing a heat source medium to heat the drying gas, and aheat sink heat exchanger containing a heat sink medium to cool andcondense liquid out of a drying gas, with a heat sink heat transfersurface comprising two or more sections connected in a functionallyparallel configuration with each other arranged to exchange heat withtwo or more substreams of the drying gas so that each drying gassub-stream exchanges heat with one of the two or more said heat sinkheat transfer surface sections per cycle through the heat exchangesystem.
 28. A heat pump for a drying apparatus including: a condenser toheat the drying gas, and an evaporator to cool and condense liquid outof a drying gas, with a heat transfer surface comprising two or moresections connected in a functionally parallel configuration with eachother arranged to exchange heat with two or more substreams of thedrying gas so that each drying gas sub-stream exchanges heat with nomore than one of the two or more said evaporator heat transfer surfacesections per cycle through the heat pump.
 29. A heat exchange apparatusoperable in a drying apparatus including a hot heat exchanger and a coldheat exchanger with two or more segments arranged such that duringoperation the segments of the cold heat exchanger lie in a functionallyparallel configuration relative to the drying gas flow, whereby two ormore sub-streams of the drying gas flow substantially exchanges heatwith no more than one of the cold heat exchanger sections per passthrough the apparatus.
 30. A heat exchange system according to any oneof claims 1 to 14 and 17 to 26 which is part of a heat pump and whereinsaid heat source heat exchanger comprises a means to evaporate the heatpump refrigerant in which at least a portion of the heat of evaporationof the refrigerant is transferred by heat exchange from a drying gasmedium, and said heat sink heat exchanger comprises a means to condensethe heat pump refrigerant after it has been compressed in which at leasta portion of the heat of condensation is transferred by heat exchange toa drying gas medium, and wherein said heat pump includes a means forsensing the wet bulb and dry bulb temperatures of the drying gas, ameans for rejecting heat from the drying apparatus, a means forcontrolling the amount of heat rejected from the drying apparatus basedon the wet bulb temperature of the drying gas such that the wet bulbtemperature is kept nominally constant for an extended period during thedrying process, and a means for controlling the total flow ofrefrigerant in the heat pump circuit based on the dry bulb temperatureof the drying gas such that the dry bulb temperature is kept withincertain limits throughout the drying process.
 31. A heat exchange systemaccording to claim 30 where the means for rejecting heat from the drierto the external environment involves full time or periodic drying gasventing, pre-cooling the drying gas entering the evaporator, pre-coolingany make-up or purge drying gas entering or leaving the apparatus,sub-cooling the liquid heat pump refrigerant after it leaves thecondenser and before it enters the evaporator, de-superheating the heatpump refrigerant leaving the compressor, and/or partially or whollycondensing the high-pressure refrigerant.
 32. A heat exchange systemaccording to either of claims 30 to 31 with said control means forchanging the rate of heat rejection from the drier based on drying timeand/or other indicator or indicators such as refrigerant temperature orpressure, moisture content of the material being dried, drying rate, wetor dry bulb temperature and/or relative humidity of the drying gas, orintegrated amount of moisture removed.
 33. A heat exchange systemaccording to any one of claims 30 to 32 with said control means forchanging the total system refrigerant flow based on drying time and/orother indicator or indicators such as refrigerant temperature orpressure, moisture content of the material being dried, drying rate, wetor dry bulb temperature and/or relative humidity of the drying gas, orintegrated amount of moisture removed.
 34. A heat exchange systemaccording to claim 33 where the refrigerant flow control means involvesreducing or increasing the heat pump compressor capacity or turning oneor more heat pump compressors on or off.
 35. A heat pump for a dryingapparatus including: a means to evaporate the heat pump refrigerant inwhich at least a portion of the heat of evaporation of the refrigerantis transferred by heat exchange from a drying gas medium, and a means tocondense the heat pump refrigerant after it has been compressed in whichat least a portion of the heat of condensation is transferred by heatexchange to a drying gas medium, and a means for sensing the wet bulband dry bulb temperatures of the drying gas, and a means for rejectingheat from the drying apparatus, and a means for controlling the amountof heat rejected from the drying apparatus based on the wet bulbtemperature of the drying gas such that the wet bulb temperature is keptnominally constant for an extended period during the drying process, anda means for controlling the total flow of refrigerant in the heat pumpcircuit based on the dry bulb temperature of the drying gas such thatthe dry bulb temperature is kept within certain limits throughout thedrying process.
 36. An apparatus according to claim 35 where the meansfor rejecting heat from the drier to the external environment involvesfull time or periodic drying gas venting, pre-cooling the drying gasentering the evaporator, pre-cooling any make-up or purge drying gasentering or leaving the apparatus, sub-cooling the liquid heat pumprefrigerant after it leaves the condenser and before it enters theevaporator, de-superheating the heat pump refrigerant leaving thecompressor, and/or partially or wholly condensing the high-pressurerefrigerant.
 37. An apparatus according to claim 35 or 36 operating withdrying gas temperatures between 25 and 90C.
 38. An apparatus accordingto any one of claims 35 to 37 using air as the drying gas.
 39. Anapparatus according to any one of claims 35 to 38 with said controlmeans for changing the rate of heat rejection from the drier based ondrying time and/or other indicator or indicators such as refrigeranttemperature or pressure, moisture content of the material being dried,drying rate, wet or dry bulb temperature and/or relative humidity of thedrying gas, or integrated amount of moisture removed.
 40. An apparatusaccording to any one of claims 35 to 39 with said control means forchanging the total system refrigerant flow based on drying time and/orother indicator or indicators such as refrigerant temperature orpressure, moisture content of the material being dried, drying rate, wetor dry bulb temperature and/or relative humidity of the drying gas, orintegrated amount of moisture removed.
 41. An apparatus according toclaim 40 where the refrigerant flow control means involves reducing orincreasing the heat pump compressor capacity or turning one or more heatpump compressors on or off.
 42. A process for drying a material using adrying gas including: causing a first sub-stream of the drying gas toflow through a heat sink heat exchanger to cool and condense liquid outof the drying gas, with a heat transfer surface arranged to exchangeheat with a first sub-stream of the drying gas, causing a secondsub-stream of the gas to flow through a heat source heat exchanger toheat the drying gas, with a heat transfer surface arranged to exchangeheat with said second sub-stream of the drying gas, said heat sourceheat exchanger being arranged in a functionally parallel with said heatsink heat exchanger so that each of said drying gas sub-streamsexchanges heat with one of the two said heat transfer surfaces per cyclethrough the heat exchange system, and causing the flow direction of thedrying gas through the heat sink and heat source heat exchangers toreverse.
 43. A drying process with a primarily closed loop recirculationof drying gas that passes over and/or through the material being driedand then over and/or through a means to cool and condense liquid out ofa first sub-stream of the drying gas and a means to heat a secondsub-stream of the drying gas such that the said drying gas sub-streamswill exchange heat by at most one of the two said heating and coolingmeans per cycle through the process.
 44. A drying process according toclaim 42 wherein at least part of the heat source heat exchanger is acondenser in a heat pump system.
 45. A drying process according to claim43 where at least part of the heat sink heat exchanger is an evaporatorin the heat pump system.
 46. A drying process according to any one ofclaims 42 to 45 including heating the drying gas to a temperaturebetween 25 and 90C.
 47. A drying process according to any one of claims42 to 46 including causing said two gas sub-streams to substantially mixafter they have passed through or around said heat sink and heat sourceheat exchangers.
 48. A drying process according to any one of claims 42to 47 including reversing the drying gas flow direction based on any oneor more of drying time, moisture content, wet or dry bulb temperature orrelative humidity of the drying gas, or integrated amount of moistureremoved from the drying gas.
 49. A drying process according to claim 48including for determining via sensors the dry-bulb and wet-bulbtemperatures and/or relative humidity for the drying gas flow enteringand/or leaving the dehumidifier.
 50. A drying process according to anyone of claims 42 to 49 including temporarily reducing the capacity ofthe heat sink heat exchanger to condense moisture out of the drying gasand/or the capacity of the heat source exchanger to heat the drying gas.51. A drying process according to any one of claims 42 to 50 includingrejecting heat from the drying apparatus to the external environmentduring the drying.
 52. A drying process according to any one of claims42 to 51 including causing the drying gas to pass over a substantiallyclosed loop path repeatedly through the heat exchange system and past orthrough a drying chamber for containing a material to be dried.
 53. Adrying process according to any one of claims 42 to 52 wherein thedrying gas is air.
 54. A drying process including producing aperiodically reversed flow of drying gas that passes through and/or overa material to be dried and through and/or over cold and hot heatexchangers arranged in functionally parallel configuration relative tothe drying gas flow.
 55. A process according to any one of claims 42 to53 including cooling and condensing liquid out of a drying gas with aheat sink heat exchanger containing a heat sink medium and a heat sinkheat transfer surface comprising two or more sections connected in afunctionally parallel configuration with each other arranged to exchangeheat with two or more substreams of the drying gas so that each dryinggas sub-stream exchanges heat with no more than one of the two or moresaid heat sink heat transfer surface sections per cycle through the heatexchange system.
 56. A drying process according to claim 55 includingcontrolling the flow of heat sink medium in the heat sink heat exchangersections to increase, decrease, turn on, and/or turn off the flow ofheat exchange medium independently in each of the heat sink heatexchanger sections.
 57. A process for drying a material including:causing a drying gas to remove moisture from the material being dried,and heating the drying gas with a heat source heat exchanger containinga heat source medium, and cooling and condensing liquid out of a dryinggas with a heat sink heat exchanger containing a heat sink medium and aheat sink heat transfer surface comprising two or more sectionsconnected in a functionally parallel configuration with each otherarranged to exchange heat with two or more substreams of the drying gasso that each drying gas sub-stream exchanges heat with no more than oneof the two or more said heat sink heat transfer surface sections percycle through the heat exchange system.
 58. A drying process accordingto claim 57 with a control system to control the flow of heat sinkmedium in the heat sink heat exchanger sections and increase, decrease,turn on, and/or turn off the flow of heat exchange medium independentlyin each of the heat sink-heat exchanger sections.
 59. A drying processaccording to claim 57 or 58 wherein at least part of the heat sink heatexchanger is an evaporator in a heat pump system.
 60. A drying processaccording to claim 59 wherein at least part of the heat source heatexchanger is a condenser in a heat pump system.
 61. A drying processaccording to any one of claims 57 to 60 including heating the drying gasto a temperature between 25 and 90C.
 62. A drying process according toany one of claims 57 to 61 including arranging the drying gas flow pathto substantially mix the two or more of the said drying gas sub streamsafter they have passed through or around said heat sink heat exchangersections.
 63. A drying process according to any one of claims 57 to 62including sensing the dry-bulb and wet-bulb temperatures, relativehumidity for the drying gas flow entering and/or leaving thedehumidifier, drying time and/or other indicator or indicators such asheat sink or source fluid temperature or pressure, moisture content ofthe material being dried, drying rate, or integrated amount of moistureremoved from the system.
 64. A drying process according to any one ofclaims 57 to 63 arranging said control system to be able to temporarilyreduce during drying, the overall capacity of the heat sink heatexchanger to condense moisture out of the drying gas and the overallcapacity of the heat source exchanger to heat the drying gas.
 65. Adrying process according to any one of claims 57 to 64 includingrejecting heat from the drying apparatus to the external environmentsuch as full time or periodic drying gas venting, pre-cooling the dryinggas entering the evaporator, pre-cooling any make-up or purge drying gasentering or leaving the apparatus, sub-cooling the liquid heat pumprefrigerant after it leaves the condenser and before it enters theevaporator, de-superheating the heat pump refrigerant leaving thecompressor, or partially or wholly condensing the high-pressurerefrigerant for purposes of control.
 66. A drying process according toany one of claims 57 to 65 arranging the drying gas to pass over asubstantially closed loop path repeatedly through the heat exchangesystem and past or through a drying chamber for containing a material tobe dried.
 67. A process according to any one of claims 42 to 53, and 55to 66 for drying a material, carried out using a heat pump and includingevaporating the heat pump refrigerant in said heat sink heat exchanger,wherein at least a portion of the heat of evaporation of the refrigerantis transferred by heat exchange from a drying gas medium, and condensingthe heat pump refrigerant in said heat source heat exchanger, after ithas been compressed wherein at least a portion of the heat ofcondensation is transferred by heat exchange to said drying gas medium,and sensing the wet bulb and dry bulb temperatures of the drying gas,and rejecting heat from the drying apparatus, and controlling the amountof heat rejected from the drying apparatus based on the wet bulbtemperature of the drying gas wherein the wet bulb temperature is keptnominally constant for an extended period during the drying process, andcontrolling the total flow of refrigerant in the heat pump circuit basedon the dry bulb temperature of the drying gas wherein the dry bulbtemperature is kept within certain limits throughout the drying process.68. A process for drying a material according to claim 67 wherein heatis rejected from the process to the external environment involving fulltime or periodic drying gas venting, pre-cooling the drying gas enteringthe evaporator, pre-cooling any make-up or purge drying gas entering orleaving the apparatus, sub-cooling the liquid heat pump refrigerantafter it leaves the condenser and before it enters the evaporator,de-superheating the heat pump refrigerant leaving the compressor, and/orpartially or wholly condensing the high-pressure refrigerant.
 69. Aprocess for drying a material according to either one of claims 67 and68 with said control for changing the rate of heat rejection from thedrier based on drying time and/or other indicator or indicators such asrefrigerant temperature or pressure, moisture content of the materialbeing dried, drying rate, wet or dry bulb temperature and/or relativehumidity of the drying gas, or integrated amount of moisture removed.70. A process for drying a material according to any one of claims 67 to69 with said control for changing the total system refrigerant flowbased on drying time and/or other indicator or indicators such asrefrigerant temperature or pressure, moisture content of the materialbeing dried, drying rate, wet or dry bulb temperature and/or relativehumidity of the drying gas, or integrated amount of moisture removed.71. A process for drying a material according to claim 70 where therefrigerant flow control involves reducing or increasing the heat pumpcompressor capacity or turning one or more heat pump compressors on oroff.
 72. A heat pump based process for drying a material comprising:evaporating the heat pump refrigerant wherein at least a portion of theheat of evaporation of the refrigerant is transferred by heat exchangefrom a drying gas medium, and condensing the heat pump refrigerant afterit has been compressed wherein at least a portion of the heat ofcondensation is transferred by heat exchange to a drying gas medium, andsensing the wet bulb and dry bulb temperatures of the drying gas, andrejecting heat from the drying apparatus, and controlling the amount ofheat rejected from the drying apparatus based on the wet bulbtemperature of the drying gas wherein the wet bulb temperature is keptnominally constant for an extended period during the drying process, andcontrolling the total flow of refrigerant in the heat pump circuit basedon the dry bulb temperature of the drying gas wherein the dry bulbtemperature is kept within certain limits throughout the drying process.73. A heat pump based process for drying a material according to claim72 wherein heat is rejected from the process to the external environmentinvolving full time or periodic drying gas venting, pre-cooling thedrying gas entering the evaporator, pre-cooling any make-up or purgedrying gas entering or leaving the apparatus, sub-cooling the liquidheat pump refrigerant after it leaves the condenser and before it entersthe evaporator, de-superheating the heat pump refrigerant leaving thecompressor, and/or partially or wholly condensing the high-pressurerefrigerant.
 74. A heat pump based process for drying a materialaccording to claim 72 or 73 operating with drying gas temperaturesbetween 25 and 90C.
 75. A heat pump based process for drying a materialaccording to any one of claims 72 to 74 using air as the drying gas. 76.A heat pump based process for drying a material according to any one ofclaims 72 to 75 with said control for changing the rate of heatrejection from the drier based on drying time and/or other indicator orindicators such as refrigerant temperature or pressure, moisture contentof the material being dried, drying rate, wet or dry bulb temperatureand/or relative humidity of the drying gas, or integrated amount ofmoisture removed.
 77. A heat pump based process for drying a materialaccording to any one of claims 72 to 76 with said control for changingthe total system refrigerant flow based on drying time and/or otherindicator or indicators such as refrigerant temperature or pressure,moisture content of the material being dried, drying rate, wet or drybulb temperature and/or relative humidity of the drying gas, orintegrated amount of moisture removed.
 78. A heat pump based process fordrying a material according to claim 77 where the refrigerant flowcontrol involves reducing or increasing the heat pump compressorcapacity or turning one or more heat pump compressors on or off.